Energetics of the naphthalene/azulene monocation isomerization: density functional and coupled cluster calculations

The energetics of isomerization between the azulene and naphthalene radical cations have been investigated using the hybrid density functional method B3LYP with the cc-pVDZ basis sets. CCSD/cc-pVDZ energy Calculations were al so carried out for selected points along the reaction coordinate. The trans ition state barrier energies for isomerization are lower than the dissociat ion limit of C8H6.+' (benzocyclobutadiene(.+)) + C2H2 deduced earlier. A ke y intermediate is a hydrogen shifted naphthalene isomer analogous to the in termediate suggested in the Dewar-Becker isomerization mechanism for neutra l azulene. The norcaradiene isomer of the Dewar-Becker mechanism was found to be a transition structure in the ionic system. Results of the present de nsity functional theory (DFT) and coupled cluster calculations are discusse d in the light of recent experimental evidence.